Physical activity is essential to prevent and treat childhood obesity, and plays an important role in the development of healthy joint function in normal weight children. Considering a greater prevalence of lower extremity malalignment in obese children, the extent to which excess adiposity, functional disabilities and abnormal gait impact the ability of this population to engage in physical activity and promote long-term musculoskeletal function and health is not yet understood. The objective of this proposal is to determine how obesity and lower extremity joint alignment affect the biomechanics (hip and knee joint contact forces) of walking and running in children. The central hypothesis is that obesity and walk/run duration negatively affect walking and running hip and knee joint loads. The rationale for the proposed research is that by quantifying walk/run hip and knee joint contact forces in obese children, we can evaluate the risk of musculoskeletal injury/pathology during these common forms of movement, allowing the development of effective weight- bearing physical activity prescriptions in the children who need it most. The first aim of this study is to determine the relationship between obesity, lower extremity alignment and hip and knee joint loading environment during walking/running in children. This will be accomplished by using musculoskeletal simulations to determine the joint loading environment during walking and running in obese and non-obese children. To establish the relationship between the joint loading environment during walking/running and osteoarticular alterations in obese children, the degree of lower extremity malalignment, determined through radiographic images, will be correlated with subject-specific joint contact force estimates. The second aim of this study is to determine how activity duration affects lower extremity joint loads and muscle forces during sustained bouts of walking and running in obese and non-obese children. By using the same musculoskeletal models developed for Aim 1, and completing similar simulations to determine joint loads and muscle forces at set intervals during prolonged walking and running, it will be possible to quantify the duration at which increased risk to the musculoskeletal system may occur in obese children. An automated treadmill controller will be used to quantity how locomotor speeds change during continuous bouts of walking and running. This project strives to better understand the connection between obesity, exercise duration, mechanical loading environment and musculoskeletal health in children. The outcomes of this research have the potential to help clinicians and physical therapists create efficacious rehabilitation treatments and guidelines and ultimately improve the overall health and quality of life of obese children.